(Modern) Shepherd

My father-in-law is a (modern) shepherd. He and a co-shepherd have charge of more than 400 sheep. Recently they treated the sheep for pests. This meant lifting each of the sheep, applying a treatment inside all four legs and to the tail -- then injecting them both sides of the neck, and inside a back leg. My father-in-law says: "Without that, a sheep is done for. It is certain death." I asked him whether they marked the sheep after treating them -- to keep track of them. "No," he said, "when a sheep has been treated, you know it. It immediately lies down."

CORRECTION: There are more than one-thousand sheep. It was more than 400 which were treated.